1. Field of the Invention
The present invention relates to an optical switch used in long-distance, large capacity optical fiber communication and the like for switching a controlled light using a control light.
2. Description of Related Art
To realize large capacity optical fiber communication using limited communication line resources effectively, means for increasing the number of channels capable of transmission and reception and means for increasing the communication speed must be provided.
In optical fiber communication, communication is performed by propagating binary digital optical pulse signals, obtained by modulating an optical pulse array in which optical pulses are arranged in series, along an optical fiber serving as a transmission line at regular fixed time intervals. The term “regular fixed time intervals” denotes time intervals corresponding to the inverse of a frequency that corresponds to the bit rate.
Multiplex communication methods such as time division multiplexing (TDM) are under investigation as such means for increasing the number of channels. TDM is a communication method in which a plurality of channels (tributary channels) is time-multiplexed and transmitted as a time division multiplex signal, and the time division multiplex signal is divided into the individual channels (tributary channels) on the reception side by a gate signal generated from a clock signal, whereby the information in the individual channels is extracted and received individually.
To increase the communication speed of the TDM described above and so on, it is desirable that all of the multiplexing/demultiplexing means be realized by optical means. In other words, it is desirable to realize an optical switch which is capable of executing a switching operation using only optical control signals rather than electrical means.
The optical Kerr effect that occurs in optical fiber is a nonlinear optical effect in which the refractive index of an optical fiber is altered when light of high intensity propagates through the optical fiber. The reaction rate of the optical Kerr effect is several femto-seconds (fs). In other words, if an optical switch is constituted using the optical Kerr effect, an optical switch that is capable of performing a switching operation at several hundred Gbit/s or more can be realized. By comparison, a conventional switch, in which an optical pulse signal is first converted into an electric pulse signal serving as an electrical signal, whereupon the electric pulse signal is switched by an electronic device and then returned to an optical pulse, is capable of switching an optical pulse signal at a maximum bit rate of approximately 40 Gbit/s.
As an optical pulse signal transmitted over an optical fiber propagates along the optical fiber transmission line, loss (so-called propagation loss) is generated in the energy of the signal. This propagation loss increases steadily as the transmission distance increases. Moreover, the chromatic dispersion of an optical fiber causes reductions in the signal to noise ratio (S/N) of the optical pulse signal and waveform distortion of the optical pulse signal, i.e. so-called waveform degradation of the optical pulse signal. Hereafter, an optical pulse signal suffering energy loss or waveform degradation will also be referred to as an optical pulse signal with deteriorated quality.
Hence, in order to lengthen the transmission distance in an optical communication device, an optical repeater is required to regenerate an optical pulse signal with deteriorated quality by performing recognition and regeneration processing on the optical pulse signal. In an optical repeater, first a regeneration step is performed to extract, from an optical pulse signal suffering from waveform degradation due to being propagated over a long-distance optical fiber transmission line, an optical clock signal having a frequency that corresponds to the bit rate of the optical pulse signal. This step is known as re-timing. Next, a step is performed to re-shape and re-generate the optical pulse signal on the basis of the optical clock signal.
A function which is capable of executing all of these three functions (re-timing, re-shaping, and re-generating) directly in an optical state, rather than by first converting the optical pulse signal into an electrical signal, is known as 3R regeneration, and a regenerator having this function is sometimes referred to as a 3R optical repeater. More specifically, a 3R optical repeater comprises a function for regenerating an optical pulse signal having an identical waveform and an identical intensity to an optical pulse signal at the time of transmission from a transmission terminal by means of optical signal processing, rather than converting the optical pulse signal into an electrical signal, and then relaying the optical pulse signal. Hereafter, a 3R optical repeater will also be referred to as an optical signal regenerating device.
With a 3R optical repeater, an optical pulse signal is regenerated by extracting an optical clock from the optical pulse signal and performing AND processing (also known as “gating”) on the optical pulse signal and optical clock using an optical gate circuit. More specifically, distortion of the optical pulse waveform of the optical pulse signal is corrected using an effect whereby, from among the optical pulses respectively constituting the optical pulse signal and optical clock, only the energy component of optical pulses superposed within the optical gate simultaneously is output from the optical gate. In this manner, the optical pulse signal is regenerated.
Hence, a 3R optical repeater performs similar signal processing to a conventional device, in which signal regeneration is performed by converting an optical pulse signal into an electric pulse signal, at a high speed and using optical signal processing technology employing only optical elements.
A conventional example of a 3R optical repeater is constituted using a nonlinear optical loop mirror (NOLM) (see “ALL-OPTICAL REGENERATION” J. C. Simon et al., ECOC' 98 Sep. 1998, pp. 467-469, for example). A NOLM comprises an optical demultiplexer/multiplexer, an optical coupler, and an optical fiber loop. An outline of the basic operations of a NOLM will now be provided.
The optical intensity of a signal light subject to switching is divided into two equal parts by the optical demultiplexer/multiplexer. The divided signal lights are propagated through the optical fiber loop in a clockwise direction (also known as CW direction) and a counter-clockwise direction (also known as CCW direction) respectively, and then re-input into the optical demultiplexer/multiplexer to be multiplexed.
If no control light is input from the optical coupler at this time, the signal light that is propagated clockwise through the optical fiber loop and the signal light that is propagated counterclockwise through the optical fiber loop are multiplexed in the optical demultiplexer/multiplexer in phase. As a result, the signal light input into the NOLM is output to the same input/output terminal as the input/output terminal from which the signal light was input into the optical demultiplexer/multiplexer. The signal light output to this input/output terminal will also be referred to as loop reflection light.
On the other hand, if a control light is input from the optical coupler, the optical Kerr effect occurs in the optical fiber loop, there by altering the refractive index. As a result, when the divided signal lights are respectively propagated clockwise and counterclockwise through the optical fiber loop, re-input into the optical demultiplexer/multiplexer, and multiplexed, the phase of the two signal lights is offset. By adjusting the length of the optical fiber loop and the intensity of the control light such that the phase shift amount equals π, the signal light input into the NOLM is output to another input/output terminal which forms a pair with the input/output terminal from which the signal light was input into the demultiplexer/multiplexer. The signal light that is output to this other input/output terminal will also be referred to as loop transmission light.
As described above, a switching operation in which a signal light serving as a controlled light is output to the-same input/output terminal as the input/output terminal from which the signal light is input into the optical demultiplexer/multiplexer, or in which the signal light is output to another input/output terminal forming a pair with the input/output terminal from which the signal light is input into the optical multiplexer/demultiplexer, is realized using a control light.
Furthermore, in a NOLM the polarization directions of the signal light propagated through the optical fiber loop clockwise and the signal light propagated through the optical fiber loop counterclockwise must be matched in order to multiplex the two signal lights in the optical demultiplexer/multiplexer. A method of constituting the optical fiber loop described above using polarization-maintaining single-mode fiber is being researched as a method for achieving this (see “Ultrafast polarisation-independent all-optical switching using a polarisation diversity scheme in the nonlinear optical loop mirror”, K. Uchiyama, H. Takara, S. Kawanishi, T. Morioka, and M. Saruwatari, Electronics Letters, vol. 28, No. 20, pp. 1864-1866, September 1992, for example).
The optical Kerr effect occurring in an optical fiber loop constituting an optical switch is dependent on the polarization state of both the control light and the signal light. More specifically, when a signal light and a control light propagate in parallel through an optical fiber loop, if both the signal light and control light are linearly polarized and the polarization planes thereof are parallel to each other, the magnitude of the refractive index change caused by the optical Kerr effect reaches a maximum. If, on the other hand, the polarization planes of the signal light and control light are orthogonal, the magnitude of the refractive index change caused by the optical Kerr effect falls to a minimum.
In an optical switch which uses the phenomenon of refractive index change caused by the optical Kerr effect, the fact that the amount of change in the refractive index depends on the relationship between the respective polarization directions of the signal light and control light leads to instability in the switching operation. It is therefore necessary to incorporate into the constitution of the optical switch a mechanism that is capable of realizing a stable switching operation at all times, independent of the polarization state of the signal light and control light input into the optical fiber loop. Here, a stable switching operation indicates a guarantee that when an optical pulse constituting a control light is input into the optical fiber loop, the signal light is invariably output from the optical switch as loop transmission light, and when an optical pulse constituting a control light is not input, the signal light is invariably output from the optical switch as loop reflection light. If a stable switching operation is not realized, the optical fiber loop cannot be used as an optical switch in a 3R optical repeater or the like.
In an optical switch used in a 3R optical repeater, the signal light described above corresponds to an optical clock signal extracted from a time-multiplexed optical pulse signal that is input into the 3R optical repeater. The optical clock signal is output from a mode-locked semiconductor laser or the like, for example. Hence, the clock signal can be set as linearly polarized light, and therefore the direction of the polarization plane thereof can be fixed and set in a desired direction as needed.
Meanwhile, in an optical switch used in a 3R optical repeater, the control light described above corresponds to an optical pulse signal, which is a time division multiplex signal input into the 3R optical repeater, or an optical pulse signal generated by amplifying and processing such an optical pulse signal using an optical amplifier or a wavelength filter. The optical pulse signal input into the 3R optical repeater is propagated through an optical fiber, which is a long-distance (approximately several tens of kilometers) transmission line. Optical fiber transmission lines, which are used widely in optical communication and the like, are constituted by optical fiber which is not capable of maintaining the polarization state of the optical pulse signal that is propagated therethrough. Hence, unless special means are provided, it is impossible to transmit an optical signal pulse over this transmission line while maintaining the polarization state of the optical pulse signal in a fixed state.
It is therefore necessary to provide an optical switch used in a 3R optical repeater with a characteristic whereby the switching operation is not dependent on variation in the polarization state of the input optical pulse signal. With a conventional optical switch using a NOLM, variation in the polarization state of the input optical pulse signal affects the switching operation such that a stable switching operation cannot be guaranteed.